This invention relates to the field of engineered thermoplastic sealing members for electrochemical cells and, more particularly, to a novel engineered thermoplastic sealing member with an affinity for absorbing electrolyte but possesses low vapor transmission (i.e., permeation) of that electrolyte through the sealing member. As used herein, the term “engineered thermoplastic” contemplates the type of materials described in Volume 2 of the Engineered Materials Handbook, first published by ASM International in 1988, and specifically includes all thermoplastics, with or without fillers or reinforcers (such as acrylonitrile-butadiene-sytrene, acetal, acrylic, fluorocarbon, nylon, phenoxy, polybutylene, polyaryl ether, polycarbonate, chlorinated polyethers, polyether sulfone, polyphenylene oxide, polysulfone, polyimide, rigid polyvinyl chloride, polyphenylene sulfide, thermoplastic urethane elastomers and other similar reinforced plastics), that have sufficient mechanical, chemical and thermal properties necessary to withstand exposure to non-aqueous organic solvents at temperature extremes potentially as great as −55° C. to 85° C. over a period of time that may be as long as 10-15 years and all without degradation that would comprise the material's ability to act as a hermetic seal for a battery housing.
The ability to withstand extreme temperature conditions, including thermal cycling and thermal shock between high and low temperatures, is becoming more important for nonaqueous cells, particularly in consumer-sized lithium batteries (e.g., International Electrotechnical Commision sizes FR6 or FR03). Specifically, transportation regulations limit total weight loss of such batteries to fractional percentages of the battery's original weight after being subjected to certain tests/conditions. However, such weight loss is typically, if not entirely, attributed to vapor transmission of the volatile nonaqueous electrolytes through and/or around the sealing mechanism(s) utilized by that housing. Not surprisingly, the more volatile the electrolyte, the greater the need for effective sealing.
In response, a wide variety of cell designs have been developed for effectively sealing nonaqueous cells. The design depends, at least in part, on the size of the cell, the type of electrode and electrolyte materials used in the cell and the power requirements of the devices to be powered by the cell. Because the cathode/electrolyte materials are so reactive, lithium cells with large liquid cathode (e.g., lithium-sulfur dioxide (Li/SO2) and lithium-thionyl chloride (Li/SOCl2)) often have housings in which metal components are hermetically welded, and glass seals are used to seal metal components that must be electrically insulated and to seal small apertures in the housings. These types of housings tend to be expensive due to the materials and the manufacturing processes and equipment required. As such, these solutions have little bearing on consumer battery systems, where cost and ease of manufacture are two of the most prominent motivations in cell design.
Other, less complex means are used to seal cells that utilize solid electrodes, including consumer-sized lithium batteries, with the more recent trend being toward engineered plastics in place of less refined or more common materials in order to improve sealing performance. Because of its relatively low cost and ease of manufacture, thermoplastic seal members are often used to effect a seal between rigid components of the cell housing (i.e., the container and closure). For example, a thermoplastic gasket or grommet can be compressed between the inside top edge of the cell container (e.g., a steel can) and the periphery of the cover which forms the closure over the open top of the can. Hermetic sealing of the container is important both to keep the active materials and electrolyte within the cell housing, as well as to keep water out.
Thermoplastic seals may also be used to effect a hermetic seal of the safety vent aperture(s) in the cell housing. Here, a conforming thermoplastic seal plugs the vent hole. Alternatively, the plug may be a rigid material, such as a glass or metal ball blocking the opening of a cylindrical thermoplastic bushing which lines the interior of the vent hole. In either case, the thermoplastic seal functions as a pressure relief vent for the cell. These arrangements are also desirable because electrolyte may be dispensed into the cell through the vent hole after the cover has already been sealed to the battery housing. Other designs which utilize a thermoplastic member to form a compressive seal between rigid parts of a rupturable vent are also possible.
FIG. 1 shows an example of a cylindrical lithium battery 10 that is common to lithium-iron disulfide cells and/or other lithium cell types (e.g., lithium-manganese dioxide, etc.). Notably, the cell design shown in FIG. 1 is also applicable to certain embodiments of the invention described herein. This design has two thermoplastic seal members—gasket 16, which in combination with top cover 40 encloses in the open end of can 12, and bushing 34, which in combination with vent ball 32 encloses vent aperture 30. Both thermoplastic seal members provide a compressive seal. Since the can 12 and cover 40 are electrically connected to opposite electrodes within the cell, gasket 16 must also provide electrical insulation between the top cover 40 and the can 12. The bushing 34 and vent ball 32 comprise a pressure relief vent for the cell. When the internal cell pressure exceeds a predetermined abnormally high level, the vent ball 32 (and sometimes the bushing 34) are forced out of the vent aperture 30, leaving an opening through which pressure is released. Cells sealed with both a gasket between the can and cover and a pressure relief vent comprising a bushing and vent plug disposed in an aperture in the cell cover are disclosed in U.S. Pat. No. 4,329,405 (issued May 11, 1982), U.S. Pat. No. 4,437,231 (issued Mar. 20, 1984), U.S. Pat. No. 4,529,673 (issued Jul. 16, 1985), U.S. Pat. No. 4,592,970 (issued Jun. 3, 1986), U.S. Pat. No. 4,927,720 (issued May 22, 1990) and U.S. Pat. No. 4,931,368 (issued Jun. 5, 1990) and U.S. Pat. No. 5,015,542 (issued May 14, 1991), the entire disclosures of which are incorporated herein.
For any cell type, the seal member material must be such that a suitable seal is maintained for an acceptable period of time and under the temperature conditions that the cell is expected to withstand during transportation, storage and use. Common characteristics of a good seal member include stability of the material in the internal cell and external environments, impermeability to the liquids and gases that are to be sealed within or outside the cell, and the formation and maintenance of a complete seal path (i.e., with no voids or gaps) at each seal interface. As noted above, consumer non-aqueous batteries are often expected to deliver service at temperatures as low as 0° C., and sometimes even as low as −40° C. or −55° C. Additionally, these cells often experience high temperatures exceeding 40° C. or, more often than not, 71° C. and higher. Thus, the seal must maintain its physical shape and integrity, while remaining non-reactive with the organic solvents, solutes and electrochemically active materials (e.g., iron disulfide, lithium, etc.), across that entire range of temperatures.
Compressibility of the seal is significant in order to allow for slight variations in manufacturing and/or component tolerances. As a result, the material used in a seal should have sufficient compressibility that can be maintained for a prolonged period of time. However, some thermoplastic materials under compressive stress tend to flow to relieve that stress. This is referred to as stress relaxation or “cold flow” of the material. Furthermore, thermoplastic materials tend to exhibit even greater stress relaxation at higher temperatures, thereby reducing the time that sufficient compression can be maintained.
Temperature also affects the compression of thermoplastic materials in another way. Different materials will expand and contract by different amounts in response to increases and decreases, respectively, in ambient temperature. Therefore, thermoplastics used to form compressive seals between more rigid components (e.g., a metal can and a metal cover) should expand and contract at close to the same rate as the rigid materials in order to maintain sufficient gasket compression over the greatest temperature range possible. If the thermoplastic is used as a compressive seal and it has a diverging coefficients of thermal expansion as compared to the rigid components in contact with the seal, unwanted gaps may form between the seal and rigid members, resulting in leaking and potential weight loss.
Thermoplastic materials available for use in nonaqueous cell seal members are more limited than for aqueous cells. Specifically, the active materials in nonaqueous cells are very reactive with water, so the seal must prevent water from permeating into the cell (as compared to aqueous cells, where water transport may be less restricted). A seal for a nonaqueous cell must also have a low vapor transmission rate for the electrolyte solvents used in the cell itself. Since the vapor transmission rate of thermoplastic material is generally dependent in part upon the vapor pressure of the solvent, low vapor transmission rates are generally more difficult to achieve for nonaqueous electrolytes containing ethers and/or other organic solvents with low boiling points common to such cell systems. Finally, and most significantly, any thermoplastic must not dissolve or degrade when exposed to the organic solvents commonly used in nonaqueous cells, thereby limiting the number of available material combinations.
Although the prior art generically teaches that a seal member's ability withstand temperature fluctuations can be improved by using engineered thermoplastic materials that maintain dimensional stability and do not crack under extreme temperature conditions, the problem of reducing electrolyte permeation and the rate of transmission of through the gasket is not addressed. This problem, as well as the electrolyte's propensity to absorb into the sealing material, is generally greater at higher temperatures and with more volatile organic solvents with lower boiling points.
Notably, as used throughout this specification, “absorb” and “absorption” refer to a material's propensity to enter into and be held by the sealing material (much as sponge would absorb water). In contrast, “permeation” is the process by which the electrolyte is absorbed into the material and then diffuses through that material such that it is released on the opposite (i.e., non-sealed) side. Numerous references teach that absorption and/or permeation are unwanted characteristics in a sealing material. For example, U.S. Pat. No. 4,333,995 discusses the disadvantages of electrolyte absorption because it causes deterioration of gasket elasticity which, in turn, leads to electrolyte leakage. Similarly, U.S. Pat. No. 5,462,820 recommends polypropylene as a non-aqueous battery gasket material because it does not swell or dissolve in organic solvents, with the swelling presumably an undesirable trait caused by electrolyte absorption.
Polypropylene is commonly used a material for lithium cell (e.g., Li/MnO2 and Li/FeS2) gaskets. Gaskets have been made with other thermoplastic materials for the purpose of improving the ability of the cell to withstand higher temperatures than with polypropylene.
Sano et al., in U.S. Pat. No. 5,624,771, disclose the use of polyphenylene sulfide (“PPS”), rather than PP, as a gasket material for a lithium cell to improve resistance of the cell to high temperatures. PPS was used to reduce gasket deformation due to cold flow under the high compressive load conditions the gasket was subjected to in the cell. However, a blown asphalt sealant was required and glass filler and elastomer content both had to be maintained at less than 10 wt. % of the material in order to prevent leakage of electrolyte through the resulting gaskets.
In U.S. Pat. No. 5,656,392, PPS and tetrafluoride-perfluoroalkyl vinylether copolymer (PFA) are identified as suitable for making a gasket for a cell that is useable at high temperatures. Here again, the addition of a glass fiber filler to the resin (to extend the stability of the gasket configuration) small amounts of PE and/or PP (to extend the temperature range that can be tolerated by the cells on a cyclic thermal shock test) are taught. But as above, gaskets containing more than 10 weight percent glass fiber were undesirable because cells made with such highly filled thermoplastic materials leaked on a temperature cycling test. The addition of more than 10 weight percent of PE and/or PP was also undesirable because of cell leakage and a continuously usable temperature of less than 150° C. for the gasket. The use of an asphalt sealant is also preferred/required.
In U.S. Pat. No. 6,025,091 Kondo et al. disclose a cell with a metal can sealed with a metal terminal cap and a gasket comprising polybutylene terephthalate (“PBT”). The gasket material can be PBT alone, PBT mixed with another polymer or PBT reinforced with inorganic materials such as glass fibers, glass beads and certain organic compounds. Kondo et al. disclose that the invention solves the problems of creeping and cracking of the gasket material when the cell is exposed to high temperature. The preferred cell type was a secondary cell, either with an alkaline or nonaqueous electrolyte (e.g., a lithium ion cell). A particularly preferred electrolyte contained LiCF3SO3, LiClO4, LiBF4 and/or LiPF6 dissolved in a mixed solvent comprising propylene carbonate or ethylene carbonate and 1,2-dimethoxyethane and/or diethyl carbonate and 1,2-dimethoxyethane and/or diethyl carbonate.
In the mid-1980's Union Carbide Corp. also manufactured a 1/3 N size Li/MnO2 cell (Type No. 2L76) with a gasket made from PBT (GAFITE® from GAF Chemicals). These cells had a spiral wound electrode design and contained an electrolyte with comprising a mixture of lithium perchlorate and lithium trifluoromethanesulfonate salts in a solvent containing 50 volume percent each of propylene carbonate and 1,2-dimethoxyethane.
When a pressure relief vent for the cell is incorporated into the seal member, the characteristics of the thermoplastic seal member that affect the operation of the pressure relief vent must also be considered when selecting a suitable thermoplastic resin. Ethylene-tetrafluoroethylene copolymer (“ETFE”) is commonly used for vent bushings in consumer Li/FeS2 cells with pressure relief vent designs similar to that in FIG. 1. When the internal cell pressure reaches a predetermined level, the vent ball or the vent ball and the vent bushing are forced outward to create an opening in the cell. When tested on a thermal shock test, ETFE-only bushings can sometimes undergo sufficient stress relaxation to cause a partial or complete loss of compression between the vent ball and cover or cause activation of the pressure relief vent undesirably low internal cell pressures.
Another approach to minimizing weight loss during thermal cycling is through the selection of electrolyte. Both U.S. Pat. Nos. 5,624,771 and 5,656,392 teach that high boiling point solvents, such as γ-butyrolactone (boiling point 202° C.) and propylene carbonate (boiling point 241° C.), can be used as electrolyte solvents to achieve the desired high temperature cell performance, either alone or in combination with the gasket materials mentioned above. Notably, these solvents can maintain practical low temperature (−20° C.) cell operation in a Li/(CF)n coin cell, and because of their low-volatility, they are at a lesser risk of volatilizing and/or absorbing and permeating through the sealing material (obviously, all solvents contemplated herein will neither absorb nor permeate through the metal container/housing used in nonaqueous-type batteries). However, electrolytes containing a large amount of low boiling point solvents such as these do not perform as well on high power discharge in lithium batteries, which can be a significant disadvantage for high power discharge applications.